This invention relates to a method of drying metal structures by applying high frequency energy such as dielectric energy and microwave radiation.
A long-standing problem with forming of plastically deformable materials which use liquids as part of the plasticizing system has been the delicate balance required to obtain material which is plastic enough to form with relative ease, and which is at the same time self-supporting and easy to handled in the wet or green state. In general, self-supporting pieces are obtained by drying. As liquid is removed, the strength of the formed body tends to increase and the body becomes more self-supporting and easier to handle. The most common method for removing liquid from formed bodies is the application of heat. A particular problem with the application of heat to such formed pieces is that surface defects, such as cracks, checks and fissures, tend to develop when heat is applied. This is because heat tends to dry the immediate surface nearest the heat source causing the formation of a skin which in turn tends to retard further evaporative drying of the body. Thus, as more heat is applied, the skin becomes thicker, trapping liquid inside the body. As the temperature increases, the trapped liquid approaches its boiling temperature, and formed vapors escape forming fissures, cracks, and other surface defects.
Certain honeycomb structures are produced from very fine metal, ceramic, glass and carbon powders and a high level of binders and water. Typically, due to the large surface area of the fine powders, the mixture requires the addition of a large amount of water sometimes more than 150 weight % based on the weight of powders in the matrix. As a result of the high water content of the matrix, structures formed from the matrix require very long drying times tend to develop cracks and fissures during the drying operation due to differential shrinkage. Consequently, the dimensional stability of such structures has remained largely unacceptable, resulting in a large number of such structures being rejected in the drying operation. This problem is particularly severe when forming complex structures such as thin-walled, honeycomb structures formed by extrusion.
Several methods have been suggested for improving the dimensional stability of formed structures. For example, it has been suggested to use high intensity energy or electromagnetic radiation, such as microwave, dielectric etc., as well as conventional drier to partially dry formed carbon articles. After forming, the wet formed carbon structure is contacted with microwave radiation to quickly remove a portion of the water in the matrix. However, after a certain critical amount of the water in the matrix has been lost, arcing occurs and if the radiation is not discontinued, the structure is eventually destroyed. After contacting the body with microwave radiation to remove a certain amount of the water, it is placed in a conventional oven at 60.degree.-150.degree. C., to substantially complete the drying. The critical level of water loss at which arcing will occur will depend on several factors including the initial amount of water in the matrix as well as the size and composition of the formed article.
One recognized method of drying ceramic bodies is by the application of dielectric or high frequency energy. For example, U.S. Pat. No. 4,439,929 discloses an apparatus for drying ceramic green honeycomb bodies using two facing electrodes having a space between them through which the green ceramic bodies may be passed while a dielectric heating energy source is connected to the electrodes and activated. An electric field, which encompasses the green ceramic honeycomb body is created within the space between the electrodes. The electric field then causes the creation of heat energy within the green ceramic body by excitation of the polar water molecules included within the body. The heat energy generated within the body raises the temperature of the body to the evaporation point of the liquids, thereby causing drying of the body.
U.S. Pat. No. 3,953,703 (Hurwitt) discloses a method for drying ceramic tape using high frequency energy, preferably in the microwave range, to uniformly raise the temperature of the ceramic slip which has been cast on a belt to evaporate volatile solvents from the interior of the tape without forming a barrier skin on the surface of the thin layer of slip.
U.S. Pat. No. 3,372,445 (Maurer et al.) describes a press for drying ceramic and other materials in the plastic state which involves predrying the extruded ceramic material prior to its separation from the press. Maurer's press includes a minimum of three axially elongated electrodes connected to a source of polyphase electric current and arranged such that extruded material, particularly ceramic material in the plastic state, is passed through the space in the center of said electrodes for the purpose of predrying the material prior to its separation from the press. Maurer et al. teach that extruded ceramic material, which passes through the space surrounded by the mentioned electrodes connected to a polyphase current source, is heated and dried by the electric field created by the electrodes. While the use of high frequency energy has proved useful for the drying of ceramic bodies, high frequency energy is not a practical method for drying electrically conductive structures particularly metals.
It is well known that metals are not compatible with high frequency energy such as dielectric and microwave energy. Recently, a method has been disclosed for uniformly heating plastically deformable material containing electrically conductive particulate matter, by induction heating. Induction heating is accomplished by placing the plastically deformable material in proximity with an induction device through which an electric current of appropriate frequency is passed, thus causing induction of an electric current or hysteresis loss within the material. Induction causes hysteresis loss within the electrically conducting particulate matter and generates heat which then uniformly heats the plastically deformable material causing it to stiffen or rigidify.
U.S. Pat. No. 3,352,951 (Sara) discloses a method of forming high density refractory carbide article by encapsulating the article within electrically conductive material having a higher melting point than the article. The encapsulated article is then sintered at a temperature just below the melting point of the article. The sintering occurs under an inert atmosphere and is accomplished by inductive heating of the electrically conductive capsule which surrounds the formed carbide article. However, induction heating is not an efficient method of drying because of the relatively low frequency range (100-3000 Hz). Further, application of inductive heat to an article formed of plastically deformable material may be disadvantageous since it is difficult to distribute the heat quickly and evenly throughout the body. Slow heating leaves the forming process with the same problem of sagging, collapsing articles. Differential heating, across the body, may lead to problems such as differential shrinkage, skin formation in the immediate vicinity of the applied heat which in turn leads to various surface defects such as cracks, fissures, or checks. Differential heating may also cause deformation of the formed body by developing opposing compression and tension forces, the tension forces being developed by faster shrinkage of the outside of a formed body.
Other methods for stiffening plastically deformable material have been suggested involving application of radio frequency energy to a formed article. However, application of radio frequency energy in many cases, causes severe problems when applied to bodies comprising electrically conductive materials, particularly metals because even moderate exposure to such radio frequency is likely to cause arcing and destroy the body. It is well known that metals are not compatible with high frequency energy. Metallic articles or articles formed from particulate metal-containing material, and which are subjected to radio frequency energy tend to be pyrophoric, particularly when very small particles of metal are used in the material since the small particles are more prone to rapid oxidation. Exposure to radio frequency energy, for more than a few seconds, leads to preferential edge heating of the formed body which is then followed by rapid oxidation and likely ignition of the material. Thus, exposure of metal particle containing formed bodies to RF energy is likely to cause severe burning unless the time of exposure is limited by a time consuming and impractical process of sequential on-off operation of the RF device.
To the best of our knowledge, to date, there have been no teachings or suggestions to dry metal articles using high frequency energy such as dielectric and microwave energy. Accordingly, it is the object of the present invention to provide an apparatus and method of drying metal ware using high frequency energy while avoiding the above recited problems.